High-strength, translucent mica glass-ceramics

ABSTRACT

The invention relates to a glass ceramic in which mica and ZrO 2  are present in crystallized form, containing: 
     K 2  O: 0-9% by weight 
     Na 2  O: 0-9% by weight with the condition that Na 2  O and K 2  O together make up at least about 4% by weight, 
     SiO 2  : 35-60% by weight 
     MgO: 10-25% by weight 
     Al 2  O 3  : 7-30% by weight 
     ZrO2: 4-12% by weight 
     F -  : 2-10% by weight which is essentially free of lithium, calcium, strontium and barium, to a process for its production and to its use as a tooth replacement.

The present invention relates to mica glass ceramics which, because oftheir mechanical properties, are suitable for a wide range ofapplications, one example of which is a tooth replacement.

Mica glass ceramics are generally known for the fact that they can beprocessed by cutting. The processibility is made possible by the foliatemorphology of micas and the fact that they are easy to cleave.

Like natural micas, the mica crystals in glass ceramics have a broadrange of compositions. The intermediate-layer position in the micastructure can be occupied by elements of the alkali and alkaline-earthgroups. While occupation by potassium or other heavy alkali elementscorresponds more to the frequently occurring natural compositions, it isalso possible for micas having alkaline-earth and lithium ions in theintermediate position to crystallize in glass ceramics. However, apartfrom micas containing Mg, they exhibit the property of appearing to haveonly limited use for technical applications. On contact with water, themica structure swells and the glass ceramic inevitably disintegrates [S.N. Hoda, G. H. Beall: "Alkaline earth mica glass-ceramics" in Adv. inCeramics, Vol. 4, Nucleation and Crystallization in Glasses, Am. Ceram.Soc, Columbus Ohio, 1982]. Full occupation of the tetrahedral positionof the mica structure by silicon leads to tetrasilicate micas whichlikewise crystallize as a mica phase in mica glass ceramics.

The classical properties of mica glass ceramics permit applications inelectronics (high resistivity, low dielectric constant) and in vacuumtechnology (non-porosity). Because mica glass ceramics arebiocompatible, they are also used in medicine as a material for bonereplacement.

In many cases, however, the comparatively low strength of mica glassceramic conflicts with the use of this material. Improved strength, inconjunction with good processibility, at least in the desired phases ofthe production process, would considerably widen the range ofapplication of mica glass ceramic.

The use of mica glass ceramics in restorative dentistry forms part ofthe background art. EP 0 083 828 B1 describes tetrasilicate mica glassceramics with increased resistance to discoloration (through theaddition of Al₂ O₃ and ZrO₂) for use as dental structure. Thecorresponding glass ceramic can be used as a prefabricated block in astandard CAD/CAM method in order to make inlays or onlays from it.

The object of the present invention is to provide a mica glass ceramicwhich has good chemical stability and high mechanical strength, a highdegree of translucence and a degree of processibility which issufficient to allow it to be processed on CAD/CAM machines.

This object is achieved by a glass ceramic in which mica and ZrO₂ arepresent in crystallized form, containing:

K₂ O: 0-9% by weight

Na₂ O: 0-9% by weight

with the condition that Na₂ O and K₂ O together make up at least about4% by weight,

SiO₂ : 35-60% by weight

MgO: 10-25% by weight

Al₂ O₃ : 7-30% by weight

ZrO₂ : 4-12% by weight

F⁻ : 2-10% by weight

which is essentially free of lithium, calcium, strontium and barium.

The proportion of F⁻ is preferably 4-10% by weight.

"Essentially free" is in this case intended to mean that said elementsshould be present at most in such proportions as are caused by theintroduction of impurities, typically overall less than 1% by weight,preferably less than 0.5% by weight. Above all, the proportion oflithium should preferably be less than 0.1% by weight, since, because ofthe low molecular weight of Li, a quantity of, for example, as little as1% by weight of Li₂ O has a considerable effect on the properties of theglass ceramic, in particular its resistance to hydrolysis.

Further, extra components for imparting color, for fluorescence or forimproving the processing properties of the glass may be added, forexample: CeO₂, La₂ O₃, MnO₂, Fe₂ O₃, TiO₂, Y₂ O₃ and B₂ O₃ in quantitiesof up to about 5% of each component, and together no more than about10%. The mixtures are produced in the corresponding proportions from theraw materials (oxides, fluorides,. carbonates, etc.). The mixtures aremelted at 1300-1600° C., preferably 1400-1550° C., for, for example, 1-5hours in a covered crucible while being stirred, then cast incorresponding molds and subsequently cooled to room temperature.

The subsequent conversion of the glass into a glass ceramic takes placeby continuous heating of the glasses to crystallization temperature orby heating to a nucleation temperature and subsequently increasing thetemperature to crystallization the glasses. The nucleation temperatureis about 20-200 K above the transformation temperature, and thenucleation time is about 0.5-3 hours. The crystallization temperature isabout 200-450 K above the transformation temperature, and thecrystallization time is about 0.5-5 hours.

At least mica crystals and ZrO₂ crystals are crystallized during thedescribed heat treatment. It is also possible for secondary phases(cordierite, enstatite, spinel) to crystallize in small quantities.

A high degree of translucence is achieved by the formation of veryfinely crystalline micas having grain sizes of 0.5-3 μm. Thehouse-of-cards structure of these very fine micas ensures sufficientprocessibility.

The strength of the glass ceramic is essentially ensured by thefine-grained micas and the conversion strengthening by tetragonal ZrO₂.Tetragonal ZrO₂ can undergo stress-induced martensitic conversion intothe monoclinic modification. This conversion is associated with anincrease in volume by 3-5 percent. As a result, the tip of an incipientcrack is placed under pressure and the strength of the material isincreased. Other ZrO₂ strengthening mechanisms include microcrackstrengthening by the formation of microcracks at spontaneously convertedZrO₂ grains and consolidation by compressive surface stresses which maybe produced by the conversion of the near-surface tetragonal ZrO₂ intothe monoclinic modification (for example by grinding). According to theinvention, it is in this case established that the useful increases instrength can preferably be achieved by the presence of tetragonal ZrO₂crystallites in the size range of from about 20 to about 200 nm.

The mica glass ceramics according to the invention can be processed invarious ways.

It is possible to grind prefabricated blocks of the glass ceramic oncorresponding CAD/CAM grinders in order to obtain a preciselydimensioned work-piece (see Examples 1 and 2). If better processibilityof the glass ceramic is required, the glass ceramic may, in an initiallyheat-conditioned and readily processible state, be processed by cutting,and the work-piece which is obtained in this way can be consolidated bya subsequent heat treatment, in which case the processibility is reducedbut the abrasion resistance at the same time increases (see Example 3).

It is further possible to melt glasses of these compositions and castthem in molten form in a corresponding mold (in the field of dentistry,for example, tooth crowns or bridges) and subsequently to crystallizethem to form a glass ceramic.

On account of its mechanical properties, the glass ceramic according tothe invention is suitable not only for inlays and onlays, like thematerials in EP 0 083 828 B1, but can further, for example, also be usedfor other forms of tooth replacement (crowns and bridges) since, besidesthe requisite processibility, for example, in CAD/CAM equipment, it alsohas high mechanical strength and, with the aid of suitable additives,may also have translucence and coloration corresponding to dentalenamel. It further has the chemical stability needed for use as a toothreplacement.

Illustrative embodiments:

Some illustrative embodiments for the novel mica glass ceramics will begiven below.

The compositions are given as weighed. The analyzed compositions arefurther given for some of the glasses. The described strengths weredetermined using the 4-point bending test (40 mm lower, 20 mm uppersupport separation) on samples measuring 3×4×≧45 mm³. The processibilityof the samples means that they can be sawed even using a commerciallyavailable hardened steel saw blade. The translucence was determinedqualitatively using 3 mm thick samples. In this case, "good" means thata black line on a white background can still be seen through a glassceramic plate.

EXAMPLE 1

Composition (% by weight)

SiO₂ : 51.58

A1₂ O₃ : 15.5

MgO: 10.42

MgF₂ : 8.9

Na₂ O: 3.5

K₂ O: 2.7

ZrO₂ : 7.4

Melting temperature 1500° C./3 hours

1st thermal conditioning: Heating rate 4 K/min, 700° C., 1.5 hours and970° C., 2.5 hours.

Strength: 265 MPa (+27 MPa)

Translucence: good

Processibility: good

EXAMPLE 2

Composition (% by weight)

SiO₂ : 53

Al₂ O₃ : 10.5

MgO: 11.5

MgF₂ : 10.3

Na₂ O: 1.5

K₂ O: 6.2

ZrO₂ : 7.0

Melting temperature 1500° C./3 hours

1st thermal conditioning: Heating rate 5 K/min, 720° C., 1.5 hours and1030° C., 2 hours.

Strength: 290 MPa (+31 MPa)

Translucence: good

Processibility: good

EXAMPLE 3

    ______________________________________                                        Composition                                                                   (% by weight)  Analysis (% by weight)                                         ______________________________________                                        SiO.sub.2:  45.04  n.a.                                                       Al.sub.2 O.sub.3:                                                                         23.42  22.7                                                       MgO:        10.35  17.4*                                                      MgF.sub.2:  10.18  4.94 (F.-)                                                 Na.sub.2 O:  5.01  5.12                                                       ZrO.sub.2:  6.0    5.94                                                       ______________________________________                                         *)analyzed as Mg and calculated to MgO                                   

Melting temperature 1520° C./2.5 hours

1st thermal conditioning: Heating rate 5 K/min, 680° C., 2 hours and900° C., 1 hour.

Strength: 205 MPa (±46 MPa)

Translucence: very good

Processibility: good

2nd thermal conditioning: Heating rate 5 K/min, 1000° C., 1 hour.

Strength: 268 MPa (±35 MPa)

Translucence: good

Processibility: poor

Alternative 2nd thermal conditioning:

Heating rate 5 K/min, 1000° C., 3 hours.

Strength: 308 Mpa (±19 Mpa)

Translucence: good

Processibility: poor

We claim:
 1. Glass ceramic in which mica and ZrO₂ are present incrystallized form, comprising:K₂ O 0-9% by weight Na₂ O 0-9% by weightwith the condition that Na₂ O and K₂ O together make up at least about4% by weight, SiO₂ : 35-60% by weight MgO: 10-25% by weight Al₂ O₃ 7-30%by weight ZrO₂ : 4-12% by weight F⁻ : 2-10% by weight having less than0.5% by weight of lithium, calcium, strontium, barium and B₂ O₃ andadditionally containing at least one oxide, selected from CeO₂, La₂ O₃,MnO₂, Fe₂ O₃, and Y₂ O₃ in quantities of up to about 5% by weight ofeach oxide and in a total proportion of not more than 10% by weight. 2.Glass ceramic according to claim 1, additionally containing componentsfor imparting color, for fluorescence or for improving the processingproperties of the glass.
 3. A process for producing the glass ceramic ofclaim 1, comprising mixing starting substances which will produce aglass ceramic havingK₂ O 0-9% by weight Na₂ O 0-9% by weight with thecondition that Na₂ O and K₂ O together make up at least about 4% byweight, SiO₂ : 35-60% by weight MgO: 10-25% by weight Al₂ O₃ 7-30% byweight ZrO₂ : 4-12% by weight, and MgF₂ : 2-10% by weight,melting themixed starting substances to about 1300-1600° C., cooling and,subsequently bringing, the molten glass slowly and/or essentiallycontinuously to its crystallization temperature.
 4. The processaccording to claim 3, in which the starting substances are selected fromthe group consisting of fluorides, oxides, carbonates and other saltswhich are converted into one or more of said oxides at hightemperatures.
 5. A process for producing the glass ceramic of claim 1,comprising mixing starting substances which will produce a glass ceramichavingK₂ O 0-9% by weight Na₂ O 0-9% by weight with the condition thatNa₂ O and K₂ O together make up at least about 4% by weight, SiO₂ :35-60% by weight MgO: 10-25% by weight Al₂ O₃ 7-30% by weight ZrO₂ :4-12% by weight, and MgF₂ : 2-10% by weightmelting the mixed startingsubstances at about 1300-1600° C., cooling and, bringing the moltenglass slowly and/or essentially continuously to a nucleation temperatureand subsequently to its crystallization temperature.
 6. A toothreplacement or part thereof comprising a glass ceramic according toclaim
 1. 7. A tooth replacement or part thereof comprising a glassceramic in which mica and ZrO₂ are present in crystallized form,comprising:K₂ O 0-9% by weight Na₂ O 0-9% by weight with the conditionthat Na₂ O and K₂ O together make up at least about 4% by weight, SiO₂ :35-60% by weight MgO: 10-25% by weight Al₂ O₃ 7-30% by weight ZrO₂ :4-12% by weight F⁻ : 2-10% by weight having less than 0.5% by weight oflithium, calcium, strontium, barium and B₂ O₃.
 8. A tooth replacementaccording to claims 7, wherein said glass ceramic is processed and/orshaped by a cutting process.
 9. A tooth replacement according to claim7, wherein an initial heat-conditioned glass ceramic shaped body isprocessed and set by subsequent heat conditioning.
 10. A toothreplacement according to claim 7, wherein said glass ceramic is producedfrom a molten glass being cast in a refractory mold and subsequentlysubjected to a heat treatment therein.
 11. Glass ceramic in which micaand ZrO₂ are present in crystallized form, comprising:K₂ O 0-6.2% byweight Na₂ O 0-9% by weight with the condition that Na₂ O and K₂ Otogether make up at least about 4% by weight, SiO₂ : 35-60% by weightMgO: 10-25% by weight Al₂ O₃ 7-30% by weight ZrO₂ : 4-12% by weight F⁻ :2-10% by weight having less than 0.5% by weight of lithium, calcium,strontium and barium and additionally containing at least one oxide,selected from CeO₂, La₂ O₃, MnO₂, Fe₂ O₃, and Y₂ O₃ in quantities of upto about 5% by weight of each oxide and in a total proportion of notmore than 10% by weight.
 12. A process for producing the glass ceramicof claim 11, comprising mixing starting substances which will produce aglass ceramic havingK₂ O 0-6.2% by weight Na₂ O 0-9% by weight with thecondition that Na₂ O and K₂ O together make up at least about 4% byweight, SiO₂ : 35-60% by weight MgO: 10-25% by weight Al₂ O₃ 7-30% byweight ZrO₂ : 4-12% by weight, and MgF₂ : 2-10% by weight melting themixed starting substances to about 1300-1600° C., cooling and,subsequently bringing the molten glass slowly and/or essentiallycontinuously to its crystallization temperature.
 13. The glass ceramicof claim 11 wherein the weight percent of K₂ O is 0%.
 14. The glassceramic of claim 11, wherein the Na₂ O is present.
 15. The glass ceramicof claim 11, wherein the K₂ O is present in an amount up to 2.7%.
 16. Atooth replacement or part thereof comprising a glass ceramic accordingto claim
 11. 17. Glass ceramic according to claim 11, additionallycontaining components for imparting color, for fluorescence or forimproving the processing properties of the glass.
 18. A toothreplacement or part thereof comprising a glass ceramic in which mica andZrO₂ are present in crystallized form, comprising:K₂ O 0-6.2% by weightNa₂ O 0-9% by weight with the condition that Na₂ O and K₂ O togethermake up at least about 4% by weight, SiO₂ : 35-60% by weight MgO: 10-25%by weight Al₂ O₃ 7-30% by weight ZrO₂ : 4-12% by weight F⁻ : 2-10% byweight having less than 0.5% by weight of lithium, calcium, strontiumand barium.